The invention relates to an information reproducing apparatus such as a disk apparatus or the like for demodulating data by an automatic equalization and a maximum likelihood detection of a reproducing channel for a disk medium or the like as a target, and to an automatic equalization maximum likelihood detecting method of such an apparatus. More particularly, the invention relates to an automatic equalization maximum likelihood detecting method of an information reproducing apparatus in which an automatic equalization and a maximum likelihood detection are executed in a lump.
In a magnetic disk apparatus, in correspondence to a high speed transfer and a high density recording, a demodulating method by a partial response maximum likelihood detection in which an equalizer and a maximum likelihood detecting circuit are combined has been put into practical use. However, inter-code interferences differ depending on variations of heads and disk media and track positions and, when an equalization error increases, an error rate deteriorates. On the other hand, when a recording density increases, a deterioration due to the equalization error further becomes remarkable and a margin for the error rate is narrowed. In order to accomplish the high density recording, therefore, it is necessary to compensate the variations by using an adaptive type automatic equalizer as an equalizer.
FIG. 1 shows an example of a conventional adaptive type equalizer. Subsequent to a pre-filter 202 for performing a (1-D) equalization to a reproduction signal, a cosine roll-off type automatic equalizing circuit 204 is provided and its equalization output is supplied to a maximum likelihood detecting circuit 206. The cosine roll-off type automatic equalizing circuit 204 constructs a transversal filter 208 by delay circuits 210-1 and 210-2, adders 212 and 214, and a multiplier 222 of a tap coefficient. However, a conventional apparatus comprising such adaptive type automatic equalizer and maximum likelihood detecting circuit has a problem such that when the number of errors increases in a code of a magnetic recording and reproducing channel, an erroneous judgment in an output of the filter frequently occurs and the equalization error doesn't decrease.